1. Field of the Invention
This invention relates to the clamping of removable micro cutting inserts onto a tool holder, such as a drill, for example.
2. Description of Related Art
A removable cutting insert is conventionally pressed against the bottom of a seat of the tool holder by a clamping screw that freely passes through a centre hole of the cutting insert in order to become engaged with a tapped hole in the bottom of the seat. However, since the two engaged threads are provided with mutual allowance in order to facilitate screwing, this allowance leads to uncertainty in the framing of the cutting insert over this bottom. Furthermore, the machined part and the vibrations are likely to apply torque to the insert around the clamping screw. Such being the case, the clamping force of the screw is insufficient for exerting sufficient pressure to prevent any transverse displacement or any rotation relative to the clamping screw, taking into account the coefficient of friction between the insert and the bottom. For this reason, the seat has a planar lateral wall, and generally even two that are respectively longitudinal and axial in relation to a general axis of the tool holder, serving as a support to the corresponding planar, lateral faces of the insert, in order to form, together with the bottom, a trihedral for receiving a corner of the insert, which is thereby blocked from moving transversely and rotating.
In its entirety, therefore, the insert has two large parallel, rectangular anterior and posterior faces joined together by four planar lateral faces defining four rectilinear edges together with the large anterior face, two successive lateral faces serving as a framing support, the two other lateral faces being capable of providing this function if the insert is reversible. Quite obviously, the seat is open axially at the end and/or laterally in order to leave one of the above edges accessible as a cutting edge.
However, for certain applications, when viewed from above, the cutting edges must have an angular-shaped profile, i.e., one side of the conventional rectangle defining the large anterior face is then replaced by two successive, unaligned segments forming, for example, a lateral point, or, in other words, the cutting edge comprises a bevelled or cut-off end portion that is turned downward in the direction of the following edge. If the insert is reversible, the cutting edge of the opposite side thus has the same angular shape. Since this shape is determined by the intersection between the large anterior face and the respective lateral supporting face, then, in theory therefore, each of the lateral supporting faces must be provided in the form of two mutually inclined planar areas, when the insert is viewed from above. In short, the conventional rectangle delimiting the large anterior face is replaced by a hexagon, and this hexagonal shape is valid for all of the sections parallel to the large faces, i.e., for the framing support sides. This becomes an octagon if the large anterior face is delimited by four edges, all of them cutting, which, in fact, are each thereby divided into two cutting edges.
The one or two supporting walls provided in the seat must therefore be adapted to the new shape of the lateral faces, i.e., each must be divided into two mutually inclined planar portions, when the seat is viewed from above. For this reason, the periphery of the trihedral receiving the insert, which is initially defined by two perpendicular lateral walls representing half of the periphery of the insert, is now defined by four lateral surfaces that admittedly still represent half of this periphery, but that now delimit the periphery of the seat according to a less angular shape, i.e., with portions or facets having an overall rounded appearance. In summary, and in order to simply matters, assuming that all of the N peripheral angles are equal, each of them is equal to 180-360/N degrees.
For a high N number, this overall rounded shape results in the fact that the bearing forces of the lateral faces of the insert on the walls of the seat are exerted with increased obliquity when resisting the parasitic torque around the screw or any other clamping means. In other words, the seat and the insert have cooperating lateral framing surfaces that, in relation to the axis of the screw, have a radially extending surface component that is now reduced, i.e., a slight extension in a direction perpendicular to the circumferential force of the parasitic torque. For this reason, the lateral walls resist the bearing force of the insert poorly, which tends to slip therein due to an increased corner effect resulting from the aforesaid obliquity, i.e., with an increase in force.
When micro-inserts are involved, i.e., inserts of a few millimetres per side, the problem of positioning them accurately and holding them is more serious, because they are used to perform high-precision machining. In the above case involving supporting faces having several facets, it often occurs that the user places the insert in an incorrect angular position, because the overall rounded appearance of the framing walls around the periphery of the seat results in the fact that they tolerate a poor assembly. Thus the user may then carry out a screwing operation, with anticipated jamming, but this screwing operation has no reframing effect, resulting in imprecise machining of the part being machined and, furthermore, due to vibrations, the insert, which is not pressed completely against the bottom of the seat, may free itself from the screw, which is not completely tightened, and thereby damage the part being machined. The tool holder and the insert may also be damaged.
Conventionally, in order to resist the parasitic torque, grooves are cut on the lateral faces, which run in the direction of the thickness of the insert, so that the area closest to the large posterior face constitutes a fastening base forming a toothed wheel with which the walls of the seat mesh, via a corresponding fluted shape. Thus, this consists in replacing an insert periphery delimited by a series of uniform, planar or rounded lateral surface profiles with a periphery having the same shape overall but corrugated, i.e., not uniform, and thus longer, which therefore has an increased number of accordion-like facets thus having a more significant radial extension in relation to the clamping screw.
A solution such as this is not suitable, due to the above-mentioned risk of poor positioning.